Permanent magnet rotors have various applications in electrical machines, including use in a permanent magnet motor as shown in FIG. 1. Permanent magnet rotors, especially those with surface mounted magnets, often require additional mechanical means for retaining the magnets on the rotor hub. In particular, at high speeds, the magnets are subject to high centrifugal forces and it is not possible to rely on adhesive bonding of the magnet to the rotor hub surface alone.
Relying on adhesive bonding on the contact surface between magnet and rotor back-iron also requires the magnet material to have sufficient strength under tension throughout its structure to prevent failure under centrifugal loads. The physical properties of sintered magnets typically used in high performance motors (e.g. neodymium iron boron NdFeB) can vary from batch to batch in production. Without resorting to X-ray inspection techniques, there is no way of identifying voids and potential crack propagation points in the material, and thus the quoted typical material properties cannot be relied upon. A suitable containment that acts to contain the magnets should they fracture in any way and prevents them from going in to tension (thus reducing the likelihood of fracture) is therefore often required. The containment must also prevent significant displacement of the magnets that would cause the rotor to become unbalanced.
Retention of the magnet blocks can also be complicated by the requirement for segmented magnets. Typical magnet materials (e.g. sintered NdFeB) are also electrically conductive. Due to armature reaction and changes in the magnet working point as it rotates past the toothed structure of the machine stator, eddy currents are induced in the material leading to losses which reduce efficiency and lead to self-heating of the magnet material which can lead to a reduction in flux density and hence machine performance due to the temperature sensitive nature of the magnet material. Therefore the magnets are often laminated or segmented in order to impede the flow of eddy currents, usually with insulating bond planes between magnet segments. This can also reduce the mechanical strength of the magnet block.
Typically, smaller rotors may have a composite sleeve placed over the magnets, as shown in FIG. 2, usually manufactured from a carbon fibre or glass fibre wound within an epoxy matrix. This may be “wet-wound” directly onto the magnet rotor and then cured. Alternatively, a tube may be pre-formed on a mandrel and fitted over the rotor. It is often desirable to achieve a pre-stress in the hoop direction of the fibres which minimises any movement of the magnets should they become dis-bonded. In the case of wet-wound rotors, this is achieved by winding the fibres under tension. For a pre-formed tube, the pre-stress may be achieved by using a thermal interference fit (e.g. rotor cooled before being dropped into a sleeve at room temperature). The outer surface of the overwrap sleeve may then be ground to achieve a high tolerance part compatible with the small airgap/clearances required in motors.
Although larger diameter machines (used in marine propulsion or wind turbines for example) often run at low to medium speeds, the combination of large diameter and speed and/or use of relatively large magnetic blocks can lead to high centrifugal forces on the magnets. However, the method of overwrapping the complete rotor with a sleeve is problematic on large diameter rotors. The pre-tension requirements to overcome the strain in the carbon-fibre under loading and maintain negligible displacement in the magnets preclude its use as the overwrap thickness required would lead to poor electromagnetic performance due to the increased magnetic airgap.
The present invention provides a method and apparatus for mounting magnets which has potential benefits in terms of ease of manufacture and mechanical strength while minimising the thickness of the retention system to ensure the magnetic airgap is minimised. It is particularly relevant to large diameter machines and high-speed applications where the centrifugal forces are significant and/or in applications with larger magnet poles.